Horst-Michael Ludwig’s research while affiliated with Bauhaus-Universität Weimar and other places

Using calcined clays as clinker substitutes in cement and additives in concretes will play a key role in achieving climate targets in the construction industry in the future. This article discusses the most important aspects of this building material. Depending on the clay minerals present, the raw clays must first be thermally or mechanochemically activated. The resulting pozzolans can generate strength-forming hydrate phases and thus provide an independent strength contribution. Compared to Portland cement hydration, forming these phases leads to a denser structure, which positively affects deformation and durability properties. Overall, calcined clays offer the potential for a very efficient, ecological building material when produced optimally. However, there is still a need for optimization in the areas of workability of fresh concrete, early strength, and carbonation.

Microstructural modelling that captures realistic features plays a key role in advancing our knowledge of concrete properties. This paper proposes a computer model based on the level set method to simulate the phase transformation during cement hydration. The model uses the level set function to reconstruct a realistic microstructure of the cement paste, specifically the fibrous form of C-S-H. The dissolution of binder phases as well as the formation of product phases such as inner and outer C-S-H, and portlandite including reaction kinetics are simultaneously reproduced. This allows the quantification of the volumetric phase changes, including chemical shrinkage as air voids. Variation of model parameters allows the investigation of geometrical interactions between phases, such as binder phases and hydrated products. The comparison between two-dimensional simulation and experimental data demonstrates that the level set based model is a promising method for modelling the microstructural evolution during cement hydration.

During the hydration of cementitious materials, C-S-H forms a layer around clinker grains, and its thickness increases over time. This layer can be differentiated into the dense inner product and the less dense, needle-like outer product. However, the measurement of this value can only be done manually at a few points without any statistical certainty. This study proposes a new method to statistically measure the hydrate layer thickness around clinker grains.

To accurately simulate the hydration process of cementitious materials, understanding the growth rate of C-S-H layers around clinker grains is crucial. Nonetheless, the thickness of the hydrate layer shows substantial variation around individual grains, depending on their surrounding. Consequently, it is not feasible to measure hydrate layers manually in a reliable and reproducible manner. To address this challenge, a software has been developed to statistically determine the C-S-H thickness, requiring minimal manual interventions for thresholding and for setting limits like particle size or circularity. This study presents a tool, which automatically identifies suitable clinker grains and and perform statistical measurements of their hydrate layer up to a specimen age of 28 days. The findings reveal a significant increase in the C-S-H layer, starting from 0.45 after 1 day and reaching 3.04 µm after 28 days. However, for older specimens, the measurement of the C-S-H layer was not feasible due to limited pore space and clinker grains.

Supplementary information for the article "A python tool to determine the thickness of the hydrate layer around clinker grains using SEM-BSE images.". The document contains additional images, graphs and tables.

To accurately simulate the hydration process of cementitious materials, understanding the growth rate of C‐S‐H layers around clinker grains is crucial. Nonetheless, the thickness of the hydrate layer shows substantial variation around individual grains, depending on their surrounding. Consequently, it is not feasible to measure hydrate layers manually in a reliable and reproducible manner. To address this challenge, a software has been developed to statistically determine the C‐S‐H thickness, requiring minimal manual interventions for thresholding and for setting limits like particle size or circularity. This study presents a tool, which automatically identifies suitable clinker grains and and perform statistical measurements of their hydrate layer up to a specimen age of 28 days. The findings reveal a significant increase in the C‐S‐H layer, starting from 0.45 µm after 1 day and reaching 3.04 µm after 28 days. However, for older specimens, the measurement of the C‐S‐H layer was not feasible due to limited pore space and clinker grains.

Concrete structures in cold and moderate climates are often exposed to a combined salt frost attack, which can cause scaling damage. Consequently, numerous test procedures were developed to determine the resistance of concrete compositions against this kind of attack. These tests mimic a severe attack with high levels of saturation, which is typical for horizontal concrete elements or structures, such as concrete pavements. Very few approaches exist for testing the salt frost scaling resistance of concretes for vertical concrete elements like bridge columns, which are subjected to only moderate levels of saturation during salt frost attack (exposure class XF2). As such elements usually didn’t show notable scaling damage in practice, this hasn’t posed a problem until recently. However, the increasing use of low carbon cements with high clinker substitution rates might increase the vulnerability against salt frost exposure for such concrete elements to some extent. To ensure adequate durability it is desirable to determine the performance of the concrete in an actual test procedure instead of relying on descriptive criteria alone. Thus, a less severe test method was developed, which is based on the CDF test. It was shown that the modified test procedure can distinguish correctly between different concrete qualities. Furthermore, the applicability of an acceptance criterion for the method was investigated.

Since the early days of concrete construction, the deterioration of concrete due to the combined action of frost, water and de-icing salts had been observed. This salt frost attack is typically associated with surface scaling. The underlying mechanism for that damage was unresolved for many decades and many conflicting damage theories exist. In the past, that knowledge gap was primarily of academic interest, as a good salt frost scaling resistance of clinker rich concretes was easily achieved with air entrainment. However, as that approach increasingly reaches its limit for “low-carbon concretes”, an in-depth knowledge of the damage mechanisms can help to identify alternative measures to ensure good salt frost scaling resistance of such concretes. Based on a novel theory, which can explain important characteristics of salt frost attack and the generation of scaling damage, an additional cause for the unfavourable influence of clinker reduction on salt frost resistance were identified.